Shazam, I don't know who told you all that, but it is plainly not true. A typical 40 gallon tank will use ~40,000 BTU/h, and a typical 50-60 gallon will use ~50,000 BTU/h. It's a very modest increase. Most 60 gal heaters will require a 4" vent; it's no big deal.
On the other hand a tankless heater may have a gas input of 200,000 BTU/h or more. The gas piping and flue venting rework is much more substantial.
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